(A) Field of the Invention
The present invention relates to a semiconductor light-emitting device, and more particularly, to a photo-electronic semiconductor device comprising P-type ohmic contacts and reflective structures.
(B) Description of the Related Art
Light-emitting diodes (LED) have been widely used in various products and have recently become an important research topic in photo-electronic semiconductor materials for producing blue light LED. Materials currently used for blue light LED include ZnSe, SiC, and InGaN etc., and those semiconductor materials exhibit band gap properties with the gap energy of approximately over 2.6 eV.
As the GaN series of light-emitting materials exhibits direct gap properties, they are able to generate light with high luminance and have the advantage of longer lifetime compared to the other similar direct gap material, ZnSe. Furthermore, the range of the gap energy of the GaN series or other III-V nitrides is between 1.9 eV and 6.2 eV, and the generated light mostly covers the band of the entire visible light and ultraviolet light spectrum. The GaN material differs from other III-V nitrides with cubic structure, wherein the crystal of GaN forms a stable hexahedron structure.
FIG. 1 is the cross section diagram of a conventional light-emitting diode 10 with the GaN semiconductor layer. An N-type semiconductor layer 12, an active layer 13, a P-type semiconductor layer 14, an ohmic contact layer 15, and a reflective layer 16 are sequentially formed on the sapphire substrate 11, and an N-type electrode 17 is formed on the exposed surface of the N-type semiconductor layer 12. The materials commonly used for P-type semiconductor layer 14 with good ohmic contact, such as nickel/gold or gold, are often poor in light reflectivity, especially for light with shorter wavelengths such as blue light, violet light, and ultraviolet light. Furthermore, most transparent conductive materials also absorb or block the light with shorter wavelengths, for example, ITO. If materials with better light reflectivity are used as the ohmic contact layer 15, for example, rhodium (Rh), palladium (Pd), and platinum (PT), although properties of good ohmic contact and fine reflectivity can both be achieved, they are mostly expensive and rare metals. Furthermore, the deposition process for the rare metals is difficult and does not result in good surface yields under general conditions of mass production, which significantly diminishes the reflective effect of light.
Generally, the resistance of the metal reflective layer 16 is far lower than that of the N-type semiconductor layer 12, and therefore, the resistance of the reflective layer 16 can be ignored and the reflective layer 16 can be considered as an equipotential level. Accordingly, the resistance of the N-type semiconductor layer 12 results in a potential gradient for the light-emitting structure in the light-emitting diode 10, where the potential distribution in the N-type semiconductor layer 12 is inversely proportional to the distance to the N-type electrode 17. As shown in FIG. 2, the horizontal distances from point O on the N-type electrode 17 to points A, B, and C on the reflective layer 16 in FIG. 1 are respectively from small to large, and therefore, the accumulative resistance of the N-type semiconductor layer 12 results in three cross-voltages Δ Vs between points A, B, and C with respect to point O from high to low. The passing current I and cross-voltage Δ V have a proportional relationship according to Ohm's law; therefore, the current passing through the light-emitting structure is non-uniformly distributed. There also is a difference in luminance that indirectly affects the efficiency and reliability of the active layer 13.
FIG. 3 is the cross-sectional diagram of another conventional light-emitting diode 30 with a GaN semiconductor layer. A dielectric layer 38 with the thickness of a quarter of a wavelength is formed on the surface of the P-type semiconductor layer 34 in the light-emitting diode 30, and a plurality of vias is formed on the dielectric layer 38. The plurality of vias is filled with metal and thereby a plurality of ohmic contacts 39 is formed. A reflective layer 36 is stacked on the dielectric layer 38, and since the reflective layer 36 typically uses gold or nickel/gold materials with poor reflectivity, the light utilization cannot be effectively improved. The thickness of the dielectric layer 38 requires good process control, namely a high-precision optical process and the related equipment, to result in an even film thickness, otherwise the original light or reflected light is not able to pass through the dielectric layer 38. Furthermore, the highly precise required process of opening micro-vias in the thick dielectric layer 38 and filling the vias with metal cannot be achieved in cost-effective mass production.
The prior art described above exhibits the disadvantages of poor light utilization, high costs, and non-uniform current distribution which cause a limited or insufficient light-emitting area.
In summary, the current market needs a semiconductor light-emitting device that can ensure low cost, stable quality, and ease of implementation to eliminate all the drawbacks of the prior art described above.